Environmental concern or other air quality control needs render desirable the desulfurization of, and removal of carbon dioxide from, gas mixtures, such as flue gases released from oil- or coal-fired steam generators or other power production plants.
Desulfurization of a gas mixture refers to removal of sulfur dioxide (SO2) from the mixture. Wet as well as dry processes for desulfurization are available. In a typical wet flue gas desulfurization process (WFGD), the flue gas is contacted with an alkali slurry in an absorption tower. In the presence of water and oxygen, SO2 reacts with the alkali to form a sulfur based salt. In some cases, the sulfur salt is precipitated as a solid. Flue gas depleted of SO2 leaves the absorption tower at the adiabatic saturation temperature, whereas the sulfur salt byproduct formed is recovered from the slurry and is used for commercial purposes or is sent to disposal. A typical dry flue gas desulfurization process (DFGD) is based on the reaction between SO2 and Ca(OH)2 in humid conditions. A humidified mixture of hydrated lime and reaction product is injected into an absorber and cools the inlet flue gas by evaporation. The cooled flue gas then flows to a dust collector where the particles in the flue gas are removed and recycled back for mixing with hydrated lime.
Removal of carbon dioxide (CO2) from a gas mixture is often referred to as CO2 capture. In a typical ammonia solvent based process for removal of CO2 from a flue gas, the flue gas enters an absorption tower where it is contacted with an absorption liquid comprising ammonia. Flue gas depleted of CO2 leaves the absorption tower whereas used absorption liquid is sent to regeneration. During regeneration, CO2 is released from the used absorption liquid. Since the absorption of CO2 from the flue gas in the absorption liquid is often operated at a temperature of the absorption liquid of below room temperature, this process may be referred to as the chilled ammonia process. During regeneration of the absorption liquid, the temperature of the used absorption liquid is raised, typically by heat exchange with steam or with the regenerated adsorption liquid returned from the regenerator, from a lower temperature employed during absorption to a higher temperature facilitating release of CO2 from the used absorption liquid.
WO 2006/022885 (U.S. patent application Ser. No. 11/632,537, filed Jan. 16, 2007, and which is incorporated by reference herein in its entirety) discloses one such method of removing carbon dioxide from a flue gas, which method includes capturing carbon dioxide from the flue gas in a CO2 absorber by means of an ammoniated solution or slurry. The CO2 is absorbed by the ammoniated solution in the absorber at a reduced temperature of between about 0° C. and 20° C., after which the ammoniated solution is regenerated in a regenerator under elevated pressure and temperature to allow the CO2 to escape the ammoniated solution as gaseous carbon dioxide of high purity.
It is of continuous interest to improve energy efficiency or reduce energy consumption related to desulfurization of, and removal of carbon dioxide from, gas mixtures. There is a need to improve the energy efficiency, such as to lower the steam consumption, involved in heating of a used absorption liquid resulting from absorption of CO2 in an absorption liquid comprising ammonia.